It is generally known that diesel internal combustion engines and lean burning gasoline internal combustion engines, in which fuel is injected directly into the combustion chamber in the cylinder, tend to produce higher NOx emissions due to the high oxygen excess. So-called selective catalytic reduction (SCR) is known for reducing the NOx content in the exhaust gas of these internal combustion engines. In the exhaust gas channel, a reducing agent is injected into the exhaust gas flow and converts the nitrogen oxides contained in the exhaust gas into harmless nitrogen and water in the presence of oxygen at the reducing catalytic converter. For example, ammonia solutions or urea solutions are added as the reducing agent.
However, the problem with these devices is that selective catalytic reduction can be reliably carried out effectively only at very high exhaust gas temperatures. Thus, at overly low temperatures in the exhaust gas line, unwanted urea deposits can occur upstream of the SCR catalytic converter; this clearly reduces the efficiency of selective catalytic reduction.
To remedy this disadvantage, a plurality of documents (DE 10 2004 004 738 A1, EP 1 712 756 A1, US 2006/0275192 A1, WO 2006/014129 A1) have already disclosed inserting into the exhaust gas channel an inner pipe, in which a static mixer is held or which acts as a type of baffle around which exhaust gas flows in the exhaust gas channel in the gap between the inner pipe and outer wall, with metering of the reducing agent taking place into the region of the inner pipe. This measure is intended to result in the site at which the reducing agent is supplied having the desired high temperature. These mechanical configurations are, however, relatively complex in terms of production technology and component engineering and lead to considerable flow losses and undesirable exhaust gas back-pressure in the exhaust gas line.
Furthermore, DE 198 56 366 C1 discloses, in order to avoid overly high temperatures in the region of the injection valve for the reducing agent for selective catalytic reduction, enclosing the injection valve by means of a double-wall valve holding body which is directly connected to the exhaust gas line so that compressed air can be blown into the air gap between the outer wall and the inner wall of the valve holding body to cool the injection valve. This results in aqueous urea solutions in particular being prevented from crystallizing out due to overly high injection valve temperatures. In conjunction with the embodiment shown in FIG. 2 of this prior art, a structure is furthermore described in which the above-described injection valve discharges into a double-wall exhaust gas channel. Sealing and insulating elements are to be provided here in a complicated manner at the orifice site of the injection valve in order to prevent the reducing agent from traveling into the gap between the two pipe walls.
The object of this invention is therefore to make available a device for post-treatment of exhaust gases of a lean burning internal combustion engine, in particular a diesel internal combustion engine, by means of which reliable and effective selective catalytic reduction of nitrogen oxides in the exhaust gas flow is possible in a manner which is simple in terms of component engineering and production technology.